Variations in the ambient temperature of a microwave resonant cavity have long been known to affect its performance, particularly its frequency response characteristics. As the ambient temperature increases or decreases, the material which forms the resonant cavity will expand or contract, thereby changing the dimensions of the cavity. Since the resonant frequency of a microwave cavity is a function of its dimensions, temperature variations will cause changes in the frequency at which the cavity will resonate.
At a given base temperature T.sub.0 the resonant frequency f of a right angle cylindrical cavity for the TE.sub.lmn or the TM.sub.lmn mode is given by ##EQU1## where D is the diameter of the cavity;
L is the effective cavity length; PA1 C is the speed of light; and PA1 X.sub.lm is the proper root of the Bessell function or its derivative according to the mode.
The incremental change in resonant frequency, .DELTA.f, due to incremental changes in the length, .DELTA.L, and diameter, .DELTA.D, in the cavity dimensions, caused by a temperature variation from T.sub.0, can be obtained from the above equation by taking its total differential, thus: ##EQU2## As the equation (2) demonstrates, frequency shift is a partial function of both cavity diameter and effective cavity length.
The problem of temperature sensitivity is evidenced in cavity filter designs at all microwave frequencies but is more pronounced at very high frequencies. Accordingly, some form of compensation must be incorporated in the filter design to achieve stability under a range of operating temperatures. The prior art solutions to the compensation problem have been generally acceptable for frequencies in the low end of the microwave spectrum, where the dimensions of the filter components are larger and impose few restrictions to compensation technique. These techniques are not satisfactory when the filters are intended for use at high frequencies such as the 11-14 GHz range employed for satellite communications. At these frequencies filter dimensions are small allowing less space for the compensation device. Accordingly, filters having a plurality of cavities often must be resorted to in order to achieve the necessary performance. However, where weight economy is a fundamental consideration such as in the design of on-board sub-systems for spacecraft, it is desirable to minimize the number of cavities required. Notwithstanding these physical limitations reliable communications' requirements demand accurate filtering of transmitted and received signals. Typical performance specifications for a satellite operating at 12 GHz allows a frequency shift of only 1 MHz. For an aluminum filter composed of several cavities operating at 12 GHz, a 40.degree. C. temperature change would result in a frequency shift greater than 10 MHz. Frequency shifts of this magnitude are unacceptable for high quality communications satellite applications. Since variations of up to 40.degree. C. in ambient temperature are typical of operating communications satellites, some form of temperature compensation is mandatory.
An additional problem is that high quality communications systems require filters having sharp frequency responses. That is, the slope of the rise and fall of the response curve should be as steep as possible making the curve flat in the band. However, as frequency increases relative channel bandwidth decreases. For example, a 5 MHz bandwidth is narrower relative to a 12 GHz frequency operating frequency than with respect to a 4 GHz frequency. This lower relative bandwidth allows more noise to enter the signal. Moreover, as frequency increases more loss occurs in the filter. In order to achieve satisfactory operation in the fundamental resonant mode of operation (TE.sub.111 for a right angle cylinder, which is a low Q or high loss mode) more filter sections must be added. This is impracticable for the confines of a spacecraft application. The alternative is to utilize a less lossy or high Q mode. For a right angle cylinder cavity, the TE.sub.011 mode is preferred at high frequencies.
Various schemes have been proposed in the prior art to compensate for the shift in resonant frequency due to temperature variation of the cavity geometry. One scheme introduces a tuning screw (or screws) into the cavity when the cavity is undergoing some dimensional change due to thermal expansion. This technique involves the insertion of a rod-like element through a cavity wall typically by way of a bi-metallic operator to affect the electromagnetic field at that location. The tuning screw alters the frequency but also causes discontinuities in the field. For example, a dielectric or metal rod would be introduced at a point where the E vector of the electromagnetic field is greatest. This forces the electric field tangential to the screw to go to zero at that point. A reduction in the total electric field of a resonant cavity will cause the frequency to decrease. U.S. Pat. No. 3,714,606, "Temperature Compensated Tuner and Oscillator," Lawrence O. Friend, issued Jan. 30, 1973 shows such a tuning screw in conjunction with a bi-metallic operator which responds to variations in ambient temperature. However, in such a device the field normal to the tuning screw is intensified and in high power applications, typically found in satellite system earth stations, voltage breakdown of the cavity medium occurs. This creates arcing-over and heat resulting in pitting of the interior cavity surface which is detrimental to optimum performance. An additional problem is that fatigue of the bi-metallic operator may reduce the lifetime of the compensation device. Furthermore, the apparatus described in the above patent is not suitable for the TE.sub.011 mode, because introduction of a tuning screw into the cavity will deteriorate portions of the E field and cause discontinuities in the TE.sub.011 mode. Although compensation by way of tuning screws is compatible with the TE.sub.111 mode, it is not useable in the TE.sub.011 mode when it is desirable to take advantage of the high Q's that mode offers.
Another technique, directed at minimizing temperature related instability rather than compensating for it, is to use a material which is substantially insensitive to variations in temperature within the range of ambient temperatures to be encountered by the filter. The most widely used of such temperature insensitive materials is Invar which exhibits exceptional dimensional stability. However, Invar is a relatively heavy metal and it is difficult to machine relative to lighter and softer materials such as aluminum or its alloys. Resonant cavity filters made of Invar, while being generally temperature insensitive, are not especially well suited for applications at high operating frequencies. Filter complexity, involving the use of multiple cavity designs, generally increases with frequency and correspondingly so does filter weight. Very high operating frequencies such as used in communications satellite would result in a filter of Invar being too bulky and heavy for efficient application. Additionally, the cost of Invar is several times greater than aluminum.
The prior art technique for temperature compensation most relevant to the instant invention involves the use of a tuning plunger having an element constructed of a material different from the cavity and/or the other plunger parts. Where the cavity is a metal such as aluminum, the plunger element generally is a dielectric such as nylon or Delrin. These materials demonstrate properties of rigidity, facile machineability and stability under most conditions. Such dielectric materials react to changes in temperature to a larger degree than the material of which the cavity and/or the other plunger parts are constructed. These plunger elements are typically in the shape of a cylindrical rod. This rod serves to vary the position of a cavity wall as a function of temperature change. One drawback with this device is the need for a carefully machined rod to operate within the close tolerances of high frequency cavity and still be structurally able to withstand the stress and vibrations encountered in spacecraft launchings. Additionally, the rod-type compensation plunger requires more operating space than that of the present invention especially at high frequencies. As is well known, as the frequency increases, the physical size of the cavity must be decreased due to the shortened wavelength. Thus the necessary length of the dielectric rod may impose a limitation on the operating frequency range of the cavity. This limitation is inconsistent with plans of expanding utilization of high frequencies. U.S. Pat. No. 3,623,146, "Temperature Compensation Cavity for a Solid State Oscillator," Yoichi Kaneko et al, issued Nov. 23, 1971 shows such a dielectric rod compensating device.
It is, therefore, an object of the present invention to provide a compact temperature compensated resonant cavity filter suitable for use in a spacecraft (with an expected lifetime equal to or greater than that of the spacecraft itself) and on the ground for high power multiplexing applications at earth stations.
It is a further object of the present invention to provide a temperature compensated resonant cavity filter which operates at the high Q, TE.sub.011 mode.
It is still a further object of the present invention to provide a low-cost temperature compensated resonant cavity filter which is simple and inexpensive to manufacture and which provides a tuning plunger which is comprised of materials having substantially different coefficients of thermal expansion.